Such adjustable-length gas springs are in particular used for adjusting tilting back rests on chairs or for adjusting so-called synchronous mechanism on office chairs, in which the seat and back rest tilting take place synchronously with one another. In connection with such chairs there is a need to be able to fix the back rest and optionally the seat shell in a given tilt position and to be able to adjust to another position. In addition, with a permanently open gas spring, it must be possible to rock the back rest and optionally the seat shell.
In the case of a gas spring of the aforementioned type known from DE-OS No. 3325798, a connecting link guide is provided for an actuating or control lever for operating the valve pin. In said connecting link guide the actuating lever can be pivoted once freely for opening and then immediate closing of the valve. It can also be fixed in the pivoting position corresponding to the open valve position by pivoting at right angles to the main actuating direction, so that the valve pin remains in its pressed in position, in which the valve is continuously open. In the case of this known gas spring, the valve is arranged at the casing end opposite to the piston rod outlet.
In an adjustable-length gas spring of the aforementioned type, such as is known from U.S. Pat. No. 4,200,332, the valve is arranged in the vicinity of the piston displaceable in the cylinder. The valve pin required for operating the valve is guided outwards through a bore in the piston rod. The permanent release mechanism of this adjustable-length gas spring is formed by a separate slider, which enables the actuating lever necessary for displacing the valve pin to be locked in its pivoted position.
European patent specification No. 0052832 discloses a permanent release mechanism for a gas spring, which is fitted as a separate component to the piston rod of an adjustable-length gas spring, whose control takes place through the hollow piston rod. This known release mechanism has a very complicated construction. It only permits a bi-stable operation, i.e. it is necessary to operate an operating knob or button twice to adjust the length of the gas spring.